Power semiconductor products are often fabricated using N or P channel drain-extended metal-oxide-semiconductor (DEMOS) transistor devices, such as lateral diffused MOS (LDMOS) devices or REduced SURface Field (RESURF) transistors, for high power switching applications. DEMOS devices advantageously combine short-channel operation with high current handling capabilities, relatively low drain-to-source on-state resistance (Rdson), and the ability to withstand high blocking voltages without suffering voltage breakdown failure. Breakdown voltage is typically measured as drain-to-source breakdown voltage with the gate and source shorted together (BVdss), where DEMOS device designs often involve a tradeoff between breakdown voltage BVdss and Rdson. In addition to performance advantages, DEMOS device fabrication is relatively easy to integrate into CMOS process flows, facilitating use in devices where logic, low power analog, or other circuitry is also to be fabricated in a single integrated circuit (IC).
N-channel drain-extended transistors (DENMOS) are asymmetrical devices often formed in an n-well with a p-well (e.g., sometimes referred to as a p-body) formed in the n-well. An n-type source is formed within the p-well, where the p-well provides a p-type channel region between the source and an extended n-type drain. The extended drain typically includes an n-type drain implanted within the n-well, and a drift region in the n-well extending between the channel region and the drain. Low n-type doping on the drain side provides a large depletion layer with high blocking voltage capability, wherein the p-well is typically connected to the source by a p-type back-gate connection to prevent the p-well from floating, thereby stabilizing the device threshold voltage (Vt). The device drain region is spaced from the channel (e.g., extended) to provide a drift region or drain extension in the n-type semiconductor material therebetween. In operation, the spacing of the drain and the channel spreads out the electric fields, thereby increasing the breakdown voltage rating of the device (higher BVdss). However, the drain extension increases the resistance of the drain-to-source current path (Rdson), whereby DEMOS device designs often involve a tradeoff between high breakdown voltage BVdss and low Rdson.
DEMOS devices have been widely used for power switching applications requiring high blocking voltages, and high current carrying capability, particularly where a solenoid or other inductive load is to be driven. In one common configuration, two or four n-channel DEMOS devices are arranged as a half or full “H-bridge” circuit to drive a load. In a half H-bridge arrangement, two DEMOS transistors are coupled in series between a supply voltage VCC and ground with a load coupled from an intermediate node between the two transistors to ground. In this configuration, the transistor between the intermediate node and ground is referred to as the “low-side” transistor and the other transistor is a “high-side” transistor, wherein the transistors are alternatively activated to provide current to the load. In a full H-bridge driver circuit, two high-side drivers and two low-side drivers are provided, with the load being coupled between two intermediate nodes.
In operation, the high-side DEMOS has a drain coupled with the supply voltage and a source coupled to the load. In an “on” state, the high-side driver conducts current from the supply to the load, wherein the source is essentially pulled up to the supply voltage. Typical DEMOS devices are fabricated in a wafer having a p-doped silicon substrate with an epitaxial silicon layer formed over the substrate, where the substrate is grounded and the transistor source, drain, and channel (e.g., including the n-well and the p-well) are formed in the epitaxial silicon. In the on-state for the high-side DEMOS device, therefore, it is desirable to separate the p-well that surrounds the source from the underlying p-type substrate that is grounded, to prevent punch-thru current between the p-well and the substrate. Although the n-well may extend under the p-well, the n-well is typically only lightly doped, and therefore does not provide an adequate barrier to on-state punch-thru current from the source to the substrate. Accordingly, a heavily doped n-buried layer (e.g., NBL) is sometimes formed in the substrate prior to forming the epitaxial silicon layer to separate the n-well from the substrate, and to thereby inhibit on-state punch-thru current from the p-well to the substrate in high-side DEMOS drivers. The n-buried layer may be connected by a deep diffusion or sinker to the drain terminal in such high-side DEMOS devices, and hence is tied to the supply voltage so as to prevent or inhibit on-state punch-thru currents.
Although the n-buried layer operates to prevent on-state punch-thru current, the NBL limits the off-state breakdown voltage rating of high-side DEMOS drivers. In an “off” state, the high-side driver source is essentially pulled to ground while the low-side driver is conducting, wherein the drain-to-source voltage across the high-side DEMOS is essentially the supply voltage VCC. In high voltage switching applications, the presence of the n-buried layer under the p-well limits the drain-to-source breakdown of the device, since the n-buried layer is tied to the drain at VCC. In this situation, the p-well is at ground, since the source is low in the off-state, and the supply voltage VCC is essentially dropped across the n-well portion extending between the bottom of the p-well and the n-buried layer, and between the channel-side of the p-well and the drain. Furthermore, as the high-side driver is shut off when driving an inductive load, the transient drain-to-source voltage may increase beyond the supply voltage level VCC.
In these situations, the lateral spacing of the drain from the p-well may be adjusted to prevent p-well to drain breakdown. However, the vertical spacing between the bottom of the p-well and the n-buried layer is more difficult to increase. One approach is to increase the thickness of the epitaxial silicon layer. However, this is costly in terms of process complexity, particularly in forming the deep diffusions to connect the n-buried layer to the drain. Accordingly, there is a need for improved DEMOS devices and fabrication methods by which increased voltage breakdown withstanding capabilities can be achieved, without increasing epitaxial silicon thicknesses and without sacrificing device performance.